This invention relates to a process for making-up polyamide resins which are solid at room temperature and which have a viscosity at 160.degree. C. of less than 3,000 mPas, a melt of the polyamide resins being cooled and cut into individual pieces.
All the viscosities mentioned in the present specification were measured by the Brookfield method.
Although polyamide resins belong to the large group of polycondensation products, they cannot be compared with high molecular weight linear polyamides. In contrast to those polyamides, polyamide resins include not only solid compounds, but also liquid compounds and have a relatively low molecular weight of up to about 10,000.
The basic types of polyamide resins are condensation products of dimeric fatty acid and ethylenediamine. They are brittle to tough, light to beige-colored products which melt at temperatures of 80.degree. to 250.degree. C. and have low melt viscosities.
Polyamide resins are used as hotmelt adhesives, printing ink binders and thixotropicizing agents. Within the context of the present invention, polyamide resins suitable as printing ink binders, which have relatively low viscosities at 160.degree. C., are of particular significance.
Printing inks for non-absorbent substrates, such as plastic films and aluminium foils, are produced from polyamide resins such as these. The resins also show excellent adhesion to Cellophane and pretreated polyolefins. They are divided into alcohol/cosolvent- and alcohol-soluble types and are used in flexographic and gravure printing. Blending with other binders provides them with certain application-specific properties. Their favorable flex, scratch and scrub resistance and the high gloss of the resulting films are particularly worth mentioning. The resins may be dissolved--according to type--in alcohol/aliphatic hydrocarbon mixtures or alcohol/aromatic hydrocarbon mixtures or in pure alcohols. Additions of esters or ketones can influence the evaporation properties of the printing inks.
The production of low-viscosity polyamide resins is generally carried out in batches by polycondensation of primary and secondary amines with dimer fatty acids. The hot melt obtained (temperature ca. 200.degree. to 300.degree. C.) has to be subsequently cooled and solidified (made up).
Various processes may be used for this purposes, depending on the type and viscosity of the polaymide resin:
Thus, underwater granulation is normally used for high-viscosity polyamide resins which are employed, for example, in hotmelt adhesives. This process and the corresponding machines are described, for example, in DE 37 02 841 C2 although the thermoplastics to be processed are not mentioned. The lower viscosity limit for the melt in this case is normally about 3,000 mPas (at 160.degree. C.). The softening point of the solid polyamide resins is in the range from about 110.degree. to 150.degree. C.
Pelletizing belts or pelletizing plates are generally used for the low-viscosity polyamide resins which are used to make printing inks. However, pelletizing belts/plates are confined to low viscosities. The viscosity range in their case is from 100 to about 3,000 mPas (temperature of the melt 130.degree. to 160.degree. C., softening point about 110.degree. C.).
The relatively old dicer technology is still occasionally used (for low to high viscosities). In this case, a ribbon is cast from the melt, cooled on a cooling belt and then cut longitudinally and transversely by the dicer. However, this technology is attended by disadvantages, such as low throughput and high energy consumption, dust and noise.
In some cases, strand granulation is also used. The machine used consists of a cooling belt similar to the pelletizing belt onto which the melt is cast in narrow, 4 to 8 mm wide strips. The spaghetti-like strands are then cooled and thus solidified. A following cutter cylinder cuts the strands into 2 to 8 mm long pieces.
In the known underwater granulation of thermoplastics, the plastic to be melted is passed through an extruder or a melt pump to a heated multiple-bore die where ti is pressed through bores arranged in a circle. A stream of water flows past the outlet side of the die. Rotating blades arranged on this side of the die cut the strands issuing from the die and partly hardened by the stream of water. The plastic granules are transported by the stream of water to a pre-drainage stage where most of the water is removed from the granules. The remaining water is then removed from the granules in a dryer. An underwater granulator suitable for this known process is described in the above-cited DE 37 02 841 C2 (Gala Industries, Inc.).
The known underwater granulation technique cannot readily be used for making up the above-mentioned polyamide resins suitable for use as printing ink binders. This is because the polyamide resins usable for underwater granulation have to have a relatively broad temperature range in which they have a viscosity of more than 3,000 mPas, but are still not so viscous that they actually solidify in the bores of the die which normally have an internal cross-section of around 3 mm. The viscosity mentioned is necessary to ensure that the strands issuing from the multiple-bore die of the underwater granulator have a sufficient consistency to be able to be cut into individual granules by the rotating blade immediately adjacent the die.
However, the viscosity of the above-mentioned low-viscosity polyamide resins usable for printing inks decreases relatively quickly with increasing temperature so that only a very narrow temperature range suitable for underwater granulation exists. If the temperature is too high, a strand which hardens as it issues from the die so that it can be cut by the blade cannot be formed; instead, the product flows uncontrollably into filaments in the stream of cooling water. Filaments and/or lumps are formed. If, by contrast, the strand is heavily precooled in order sufficiently to increase the viscosity, the product is in danger of partly solidifying in some of the outlet bores of the multiple-bore die. Since the gear pumps or extruders used to transport the melt are capable of generating a constant volumetric flow rate largely irrespective of the pressure, the blockage of some of the outlet bores results in an uncontrollable increase in throughput of product from the other bores of the die. This in turn prevents controlled drop formation and hardening of the product in the stream of cold water, resulting in the formation of filaments or tangles and lumps. A reduction in the output of the pump or the extruder only leads to further blockage of the bores. An interruption in production is unavoidable.
Accordingly, the problem addressed by the present invention was to make up low-viscosity polyamide resins by the process mentioned at the beginning using an only slightly modified machine which could also be used for making-up high-viscosity polyamide resins.